The present invention relates to a fluoroelastomer processing aid masterbatch that is formulated using a particulate matrix material, and, more particularly, to a method for dispersing a wide range of elastomers in a matrix material to achieve improved uniformity and control of elastomer particle size and improved elastomer particle size distribution in the masterbatch.
U.S. Pat. No. 3,125,547 describes the use of 0.01-2.0 wt. % of a fluorocarbon polymer that is in a fluid state at the process temperature, such as a fluoroelastomer, to reduce die pressure in extrusions of non-fluorinated polymers, such as high and low density polyethylene and other polyolefins. Use of such additives can provide for significant increases in extrusion rates without the extruded polymer developing melt fracture.
U.S. Pat. No. 6,005,025 describes a method for dispersing solid forms of additives in synthetic thermoplastic polymers where the particle size of the additive is fixed and not likely to change during processing. In the case of fluoroelastomers, however, their particle size and particle size distribution can change rather dramatically during processing at normal melt-processing temperatures.
U.S. Pat. No. 6,048,939 describes a method for producing extrudable compositions and masterbatches comprising a minor amount of fluoropolymer dispersed in a non-fluorinated host resin, by adding fluoroelastomer latex directly to the host resin. The mixture is then allowed to dry to remove the liquid component of the latex, or extruded directly so as to use the extrusion process to de-volatilize the mixture. However, the described method does not contemplate using the physical dimensions of the host resin as a means of controlling the fluoropolymer particle size in the mixture. Particle size of the host resin is not disclosed, and the fluoroelastomer latex is mixed with the host polymer at temperatures which are above the melting point of the host polymer.
U.S. Pat. Nos. 6,642,310 and 6,875,814 describe extrudable compositions comprising a non-fluorinated host resin having from 25 to 2000 ppm of a fluoropolymer dispersed therein, in which the fluoropolymer particles have a weight average diameter in the range of 2 to 10 microns. It is stated that a size range of from 2 to 10 microns enables the fluoropolymer to function more efficiently as a process aid, e.g., by reducing melt defects, such as melt fracture, in linear low density polyethylene.
It is well known that fluoropolymer process aids function by depositing a fluoropolymer coating on internal die surfaces, and that extrudable compositions containing a fluoropolymer process aid(s) exhibit preferred extrusion processability when the fluoropolymer is dispersed within a rather narrow size range, i.e., as taught by U.S. Pat. Nos. 6,642,310 and 6,875,814. However, the best currently known practice for achieving an optimal fluoropolymer size range relies on dispersing very large fluoropolymer particles (e.g., initially greater than about 100 microns in diameter) using conventional compounding techniques in which the host resin is in a liquid state. This technique relies on selecting fluoroelastomers having a narrow viscosity range, and incorporating therein an interfacial agent, such as, for example, a polyethylene glycol or polycaprolactone. (See, e.g., U.S. Pat. No. 7,001,951) Fluoroelastomer particle size distributions in such extrudable compositions tend to be very broad, having many particles smaller than optimal (i.e., less than 2 microns), and a small number of particles larger than optimal (i.e., greater than 10 microns). The smallest fluoroelastomer particles are essentially “wasted” because, being generally less than 2 microns in size, they are too small to effectively benefit extrusion processability of the composition. The largest particles have been observed to form defects in the extruded articles, particularly in cases of film extrusion.
Thus, there is a need for improved fluoroelastomer masterbatches for use as process aids that can achieve better uniformity and control of fluoroelastomer particle size and particle size distribution in the extrudable composition.